We report the construction of an optical tweezers system and the results of the trapping and the moving of living algal cells.Experimental study on trapping polystyrene spheres demonstrates the effects of the system parameters on the trap.The trapping forces on dielectric spheres are calculated using Ashkin’s ray-optics model.The result shows that an objective with higher numerical aperture can build a more stable trap along axial direction than an objective with lower numerical aperture,though their transverse trapping forces are on the same order.